1. Field of the Invention
This invention relates to a pattern generator, a pattern generating reticle and a process for generating patterns utilizing said generator and reticle. More particularly, it is applicable to the production of very large scale integration (VLSI) circuits used in the electronic circuits of information processing systems.
2. Description of the Prior Art
The techniques presently used in the production of electronic circuits of information processing systems increasingly call for the use of VLSI circuits. These integrated circuits are generally offered in the form of small rectangular or square wafers, usually called "chips", having sides measuring a few millimeters. In current practice, manufacturing technologies allow the arrangement of several tens of thousands of transistors inside each one of these chips, together with their conductive interconnecting networks.
For the sake of simplicity, an integrated circuit can be defined as a pile or stack of different layers on a silicon substrate. These different, extremely thin, layers can for example, be composed of silicon oxide, phosphosilicate glass, silicon nitrite, polysilicon, metal semiconductors or layers of aluminum. During the fabrication of integration circuits, several chips that are physically different from one another can be placed on the same silicon substrate. Thus, on a silicon substrate having dimensions that are essentially the same as those of a disk having a diameter on the order of 120 mm, one can produce several tens of chips having, for example, the form of squares on one side. When the fabrication of all of the chips arranged on a common substrate is complete, they are physically separated from one another, e.g., by sawing.
To prepare each one of the layers of the same chip, the manufacturing process thereof consists of several stages which utilize different techniques, such as deposition of material on the layer, etching, doping of the layer, oxidation of the layer, etc. For each of these different stages, it is necessary to delimit on the layer of the chip being prepared one or more geographical areas on which is defined a plurality of patterns. This operation is carried out by means of a technique usually called photolithography.
Photolithography consists, first, in coating the layer of the integrated circuit to be treated with a photosensitive resin. This resin coated layer is then lighted or illuminated by an appropriate luminous radiation, either through a mask or through a reticle bearing the representation of the group of patterns to be reproduced on the area of the layer of the integrated circuit to be treated. This latter operation is presently known as the name "exposure" operation.
During this exposure, the illuminated areas of the resin undergo a chemical transformation. There are two types of photosensitive resins, namely, positive resins and negative resins. In the case of positive resins, one exposes (illuminates by luminous radiation) the areas which shall be treated by deposition, etchings, paroxidation, etc. In the case of negative resins, the areas which shall be protected are covered. Regardless of the type of resin, the resin of the exposed areas is dissolved in an appropriate chemical bath. Thus, it is obvious that the parts of the material coated with resin and which have not been exposed remain protected by the resin. One can then proceed to the various physicochemical treatments mentioned above using this resin mask.
As described hereinabove, the resin is illuminated through a mask or through a reticle. The mask is a representation on a scale 1 of the areas of the resin sought to be protected. During the exposure, the mask is brought into direct contact with the resin coated layer of the integrated circuit.
The reticle is a representation on a scale much larger than 1 (usually equal to 10) of the patterns that shall be reproduced on the area or areas of the resin coated layer to be treated. The image of this reticle is then projected in reduced form (the reduction ratio is equal to the reverse of the above mentioned ratio, i.e. one-tenth in the case where this scale is equal to 10) onto the resin coated layer.
Thus, at least one reticle or one masking corresponds to a given layer of a chip (very large scale integration circuit).
In view of the foregoing, it is clear that patterns can be generated on layers for integrated circuits include a source transmitting a beam of light (having a wavelength bordering on near-ultraviolet, for example); for each one of the layers, at least one associated reticle bearing patterns which shall be reproduced thereon, said reticle being transparent to the light beam transmitted by the source; and optical means for projecting the image of the patterns onto the layer of integrated circuit coated with photosensitive resin.
The assembly of means defined above is called a pattern generator for integrated circuits.
The chip-bearing substrate is arranged on a table which is insensitive to vibrations and is provided with a system enabling the substrate to move in accordance with two degrees of freedom, i.e., according to vertical OX and OY axes. This table is provided with an extremely accurate positioning system and is, for example, equipped with a laser interferometer (for example, of the type manufactured by HEWLETT PACKARD under No. 5501 A and described in their technical bulletins). The pattern generator is placed on a machine provided with an extremely fine optical alignment system which enables it, by means of special sighting marks, to obtain positioning accuracies of the chip on which one desires to reproduce the patterns in relation to the generator on the order of two-tenths of a micron.
Generally speaking, in current practice, a reticle is produced in the following manner: There is produced on a glass substrate (which must be transparent to the light beam transmitted by the source) a chromium layer which is then coated with an electrosensitive resin. The definition of an electrosensitive resin is quite similar to that of a photosensitive resin, the only difference being that it undergoes a chemical transformation under the action of an electron beam and no longer under the action of light.
If the resin is positive, the patterns are written by an electron beam in such a way that the areas which shall be treated (those forming the patterns) are exposed to said beam, whereas, if the resin is negative, the areas sought to be protected are exposed to the electron beam. The electrosensitive resin is then developed in an appropriate chemical bath, the chromium is then corroded, in the portions that are no longer protected by the resin.
When the reticle is completed, it is necessary to control the patterns that have been written thereon. If for any reason there is an excess of chromium on a given location, this excess is evaporated by a laser beam. On the other hand, if a pattern of chromium is missing on a given location, the whole procedure leading to the writing of the patterns by the electron beam as described earlier must be taken up again on said location. Experience has shown that if the writing of a reticle with the aid of an electron beam is rapid (half an hour), the control thereof is very long and may take several days, which makes this latter stage extremely expensive.
The system of stages leading to the generation of patterns on a layer for very large integration circuits described above constitutes a process for generating patterns on a layer for integrated circuits, whose essential successive operations are listed on the table in Appendix I.
Therefore, a great drawback in pattern generators used in the past as well as the generating process utilizing such generators is the mode of obtaining the reticle.
According to the invention, this disadvantage is overcome by replacing the reticle formed by a chromium deposit on a glass substrate with a magnetooptical reticle having magnetooptical materials and the Faraday effect are recalled to mind in the following section.
Among the magnetic materials having magnetooptical properties one includes especially the iron garnets, yttrium and rare earths (gadolinium, terbium, etc.). These materials and their magnetooptical properties are, for example, described in a paper entitled "Large Stable Magnetic Domains" written by G. R. Pulliam, W. E. Ross, B. McNeal, and R. S. Bailey published in Applied Physics 53(3), March 1982, pp. 27 54 to 27 58. Said materials are transparent to light. For any garnet of this type and, more generally, for any magnetooptical material, the magnetooptical effect is based on the principle of interaction between a polarized rectilinear light and the magnetic state of the garnet or of the material. If this interaction takes place because of the transmission of the light through the material, the magnetooptical effect is called "Faraday effect". If it takes place through reflection, the effect is called "Kerr effect". Hereinafter, this description will be limited to the Faraday effect.
It will be recalled that a light is polarized rectinearly in the plane when the electric field vector (and, hence, the magnetic induction vector) always retains the same direction in the plane perpendicular to the direction of propagation of the radiation. The plane of polarization is defined as the plane containing the direction of propagation of the light and the electric field vector.
The result of this interaction is a rotation of the electric field vector in the plane perpendicular to the direction of propagation (i.e., in the plane of polarization).
To observe this magnetooptical effect, a polarized rectilinear (preferably monochromatic) beam of light is transmitted over the surface of the magnetooptical garnet whose magnetization is usually normal to said surface (the garnet is said to have a vertical magnetization). It is observed that after passing through the layer of magnetic garnet, the electric field vector of the polarized light undergoes a rotation in the plane of polarization which, by convention, is considered equal to an angle (-.theta.) when the light encounters an area of the garnet where the magnetization is called negative (i.e., having the same direction of propagation of light and being equal to (+.theta.) as the polarized light passes through a zone where the magnetization is positive (direction opposite to the direction of propagation of the light).
To write patterns on the magnetooptical reticle, it suffices simply to write thereon by means of a magnetic or thermomagnetic transducer of the type used currently in magnetic disk storages domains of magnetization (e.g., positive) having the form of the patterns sought to be reproduced on a given layer for a very large scale integration circuit, the remainder of the reticle surface (except patterns) having a negative magnetization.
A magnetic transducer is usually composed of a magnetic circuit around which a winding is arranged and which includes an air gap.
A thermomagnetic transducer is formed, on the one hand, by a point-shaped heat source which allows the temperature of the magnetic material to rise locally above its Curie point or its point of compensation and, on the other hand, by a system which creates a permanent magnetic field having an intensity which is sufficient to orient the magnetization of the previously heated part during its cooling.
To project the image of the patterns (formed by the magnetic domains with a positive magnetization) of the reticle onto the resin coated layer for very large scale integration circuits, the reticle is illuminated with a polarized light beam. As it passes through the domains with a positive magnetization (the patterns), the electric field vector of the light undergoes a rotation equal to (+.theta.), whereas, as it passes through the areas of the reticle external to the domain forming the patterns, the electric field vector of the polarized light undergoes a rotation in the plane of polarization which 9 is equal to (-.theta.). If there is placed between the reticle and the layer of the integrated circuit a light meter formed by a crystal oriented in its direction of propagation and arranged in such a way that said direction is at right angles to the direction occupied by the electric field vector of the transmitted light as the latter passes through a domain with a negative magnetization, a light with zero intensity is absorbed at the outlet of the light meter as the light passes through a domain with a negative magnetization, whereas a light having non-zero intensity is absorbed as the light passes through a domain with a positive magnetization (one pattern).
In other words, due to the presence of the light meter, everything happens as if only domains with a positive magnetization transmitted the light. It should be obvious that the light meter could be placed differently, so that only domains with a negative magnetization transmits the light. Thereby, one can project the image of the domains with a positive magnetization (patterns) on the layer for integrated circuits onto which one seeks to reproduce these patterns.